The present invention is directed to a method of forming titania clad alumina particulates having high resistance to sulfur degradation. The products formed by the present method provide improved catalyst supports and resultant catalysts. These catalysts include those useful for treating internal combustion engine exhaust products, especially exhaust products of diesel engines, as well as other applications, including those used in Fischer-Tropsch and other hydrogenation reactions. The method of the present invention provides an economical and commercially desirable means of forming a substantially uniform cladding of titania on alumina particulate. The product achieved by the present invention retains the high surface area and porosity of the initial alumina while substantially reducing the alumina surface not having a protective titania coating thereon from those attained by conventional impregnation processes.
The exhaust products of internal combustion engines are known health hazards to human beings, animals as well as plant life. The pollutants are, in general, unburnt hydrocarbons, carbon monoxide, nitrogen oxides, as well as residual amounts of sulfur and sulfurous compounds. These pollutants have been successfully treated by contact with multifunctional, noble metal catalysts which are capable of converting a high percentage of the pollutants into less harmful products of carbon dioxide, water (steam) and nitrogen. However, the sulfur and sulfurous compounds and agents present in fuels, especially from middle cut petroleum feedstock (C10 and greater hydrocarbons), and, in turn, in exhaust streams have been known to poison the noble metals resulting in lessening their catalytic effectiveness and life. The removal of such sulfurous materials has been deemed difficult to accomplish due to the complex nature and scope of such materials.
The sulfurous materials contained in fuels tend to oxidize in the combustion process to form sulfur oxides. In turn, these sulfur oxides may then react with the alumina supports of the catalytic materials located downstream of the engine to convert it into sulfates that reside on the surface of the aluminum oxide support. It is believed that such coverage of the alumina surface with sulfate generally causes a loss in catalyst activity. In addition, the sulfates initially residing on the alumina surface may further convert into sulfuric acid which is then carried out of the system as part of the exhaust stream.
Catalytic supports are generally formed of high porosity inorganic oxides, such as alumina and titania. For example, titania is known to have a much lower tendency to react with sulfates and, therefore, exhibits a greater durability in the presence of gaseous vapors that contain sulfurous materials and the like. However, when titania is used as a support, it does not exhibit the hydrothermal stability required to form effective emission control catalyst supports. When exposed to equivalent temperatures, titania sinters to a greater extent than alumina and, thus, causes the material to have lower surface area and, hence, lower catalytic activity than its alumina counterpart. Thus, titania, per se, does not provide a desirable catalyst support material.
In attempts to overcome the poor hydrothermal properties associated with titania, alumina supports have been coated with titania either by conventional impregnation or by co-precipitation techniques (See US Published Application 2005/0272827). In both instances, the resultant support retains a significant amount of exposed alumina because of the tendency of the titania or its precursor to be rejected by the alumina and because the titanium material, when initially deposited, acts as nucleating cite for further deposition. The result is a coating that is composed of discontinuous islands of titanium material coated over the alumina underlayment support. Conventionally coated supports still have large amounts of exposed alumina surface area and, therefore, exhibit a high degree of susceptibility to sulfur poisoning. Further, titania applied by conventional methods tends to precipitate in the pores of the alumina causing blockages of the pores and, thus, reducing the surface area upon which the noble metal may reside.
U.S. Pat. No. 4,759,918 discloses a catalytic composite deemed useful in the treatment of diesel soot. The composite comprises a sulfur resistant refractory inorganic oxide, such as alumina that has been wash coated with an aqueous slurry of preformed titania. Although the product may exhibit some resistance to sulfur degradation, the coating does not protect the entire surface of the alumina or must be applied in such large quantities that it not only covers the surface but also bridges the pore openings of the alumina.
U.S. Pat. No. 4,705,770 discloses a method of preparing an anatase titania catalyst support by forming a solution of ammonium titanyl sulfate from titanium tetrachloride and impregnating a porous substrate multiple times with the formed solution. After each impregnation, the deposited material is decomposed by treating it to high temperatures and then calcined to transform the titanium into anatase titania. The resultant product has a thick, irregular coating of titania and a substantial loss of porosity of the substrate.
US Published Application 2005/0272827 discloses a catalyst used in hydrogenation and Fischer-Tropsch reactions composed of cobalt on a titania coated support. The titania coated support is formed by conventional impregnation techniques using organotitanate compounds and organic liquids to achieve its coatings. Such processes require expensive organic starting compounds and liquids that call for special techniques and equipment (e.g. explosion proof equipment, etc.) which makes the process prohibitive for commercial application.
U.S. Pat. No. 7,169,433 discloses a process of coating a base particle, such as iron, by forming an aqueous suspension of the base particle having a pH of between 7 and 12, adding a hydrogen peroxide/ammonia solution also having a pH of from 7 to 12 and containing peroxotitanic acid. As the peroxotitanic acid solution is introduced into the suspension, it decomposes and forms a titania film coating on the base powder.
Japanese Laid Open Application 2000-345072 discloses a method of forming a blue powder having smooth surfaces. The powder is formed by first coating iron magnetite powder with silica followed by precipitation of preformed titania. The titania was initially formed from an aqueous solution of titanyl sulfate held at a buffered pH of 5.4 and elevated temperature to cause the water to hydrolyze the titanyl sulfate to titania. Very dilute amounts of the titania formed solution was then contacted with the silica coated magnetite under ultrasonic agitation to provide a coating thereon.
Vapor deposition using titanium tetrachloride has been suggested (See U.S. Pat. No. 4,459,372) to achieve a uniform coating of titania. However, such a process is not commercially desirable due to the handling problems associated with titanium tetrachloride reactant and the elaborate equipment required for vapor deposition processing. Further efforts to produce a titania cladding has included chemical vapor deposition of titania onto the surface of alumina (See US Published Application 2005/0129601; US Published Application 2003/0143421 and U.S. Pat. No. 7,022,646). Again such a method requires elaborate equipment and processing.
The present invention brings together both of the beneficial aspects of the materials forming titania clad alumina particulates and the feasible and cost effective method steps that readily forms an improved final catalyst capable of exhibiting high sulfate resistance, durability, and catalyst activity after exposure to high temperatures and processing or exhaust streams containing sulfurous materials. The present invention is directed to an economical and commercially feasible method to form a highly dispersed, substantially uniform layer of titania on a support oxide composed of the thermally stable alumina. Further, the presently achieved titania cladding has dimensions that allow it to be formed on the surface of the alumina without causing blockages of its pores to, thus, substantially retain the high surface area of the initial alumina particulate.
It is desired to provide an economical and commercially feasible method to form a titania clad alumina catalyst support capable of enhancing the activity of noble metals in the conversion of carbon monoxide and hydrocarbon materials to carbon dioxide and water while exhibiting high tolerance to the presence of sulfur and sulfurous compounds.
It is further desired to form an alumina catalyst support having titania cladding on the support's surface. The cladding should be substantially continuous over the surface of the alumina, cause no or only minor reduction of the surface area of the porous alumina (e.g. by not clogging the pores) and provide high efficiency to tolerate the presence of sulfur and sulfurous compounds and agents.
It has been presently found that alumina particulate material clad with titania formed according to the present invention provides a support for noble-metal catalysts. The resultant supports exhibit enhanced sulfur tolerance in comparison to supports having the same titania content formed by conventional impregnation or co-precipitation methods. The method of the present invention provides an economic and commercially feasible method of forming the desired titania clad alumina support product.
The method of the present invention permits the economical formation of desired alumina support and resultant catalyst for effectively treating emission product streams of internal combustion engines, especially diesel fuel engines, as well as other applications. The thin, substantially uniform nature of the titania cladding achieved by the present invention allows for improved mass transfer while not imparting bridging of the pore surfaces which would reduce the porosity of the alumina core. All of the benefits can be achieved using readily available, easily handled and processed materials and steps to, thus, provide an economic, commercially feasible method described herein below in full.